1. Field
The invention relates to surge protection. More particularly, the invention relates to a surge protection device for passing DC and RF signals.
2. Related Art
Surge protection devices protect electronic equipment from being damaged by large variations in the current and voltage across power and transmission lines resulting from lightning strikes, switching surges, transients, noise, incorrect connections, and other abnormal conditions or malfunctions. Large variations in the power and transmission line currents and voltages can change the operating frequency range of the electronic equipment and can severely damage and/or destroy the electronic equipment. For example, lightning is a complex electromagnetic energy source having potentials estimated from 5 million to 20 million volts and currents reaching thousands of amperes that can severely damage and/or destroy the electronic equipment.
Surge protection devices typically found in the art and used in protecting electronic equipment include capacitors, diodes, gas tubes, inductors, and metal oxide varistors. A capacitor blocks the flow of direct current (DC) and permits the flow of alternating current (AC) depending on the capacitor's capacitance and the current frequency. At certain frequencies, the capacitor might attenuate the AC signal. For example, the larger the capacitance value, the greater the attenuation. Typically, the capacitor is placed in-line with the power or transmission line to block the dc signal and undesirable surge transients.
Gas tubes contain hermetically sealed electrodes, which ionize gas during use. When the gas is ionized, the gas tube becomes conductive and the breakdown voltage is lowered. The breakdown voltage varies and is dependent upon the rise time of the surge. Therefore, depending on the surge, several microseconds may elapse before the gas tube becomes ionized, thus resulting in the leading portion of the surge passing to the capacitor. Gas tubes are attached at one end to the power or transmission line and at another end to the ground plane, diverting the surge current to ground.
Inductors can be attached to the power or transmission line after the gas tube and before the capacitor to divert the leading portion of the surge to ground. An inductor is a device having one or more windings of a conductive material, around a core of air or a ferromagnetic material, for introducing inductance into an electric circuit. An inductor opposes changes in current, whereas a capacitor opposes changes in voltage.
One drawback of conventional surge protection devices is the difficulty in impedance matching the surge protection device with the system. Another drawback of conventional surge protection devices is the elevated voltage at which they become conductive and the higher throughput energy levels. Still yet another drawback of conventional surge protection devices is poor bandwidth capabilities and poor RF performance at high power levels.